This invention relates to apparatus for producing an output signal upon sensing a magnetic field and, in particular, to such apparatus which is capable of sensing the proximity of a member having material of high magnetic permeability.
In many applications it is desired to produce a switching function as a function of a mechanical device, but without the use of mechanical contacts which must be engaged by and thus physically closed by the mechanical device. Such an operation generally can be achieved by a contactless switch. That is, depending upon the position of the mechanical device in question, a switching function, such as the actuation of an electronic switching element, can be performed. Generally, diverse types of contactless switches are known. Among these are photoelectric devices, electro-static devices and magnetic-sensitive devices, all of which can be used to perform a switching function without actual physical engagement of mechanical contacts.
A typical magnetic-sensitive switching element which has been used heretofore is the well-known magnetic head which is generally used as an electro-magnetic transducer for recording and playback operations in the tape recording art. When used as a switch, an electric output signal is produced as a function of a magnet which is juxtaposed to the magnetic head. Consequently, when the relative position of the magnetic head and a magnetic pole is charged, this change in position produces the electric output signal. Hence, depending upon the particular application thereof, the magnetic head can be used to produce various switching functions, accordingly.
However, the use of the magnetic head as a switch suffers from various disadvantages. One disadvantage is the relatively large structure which must be used. The magnetic head requires an electromagnetic coil element, a suitable supporting core and a bias signal oscillator in order for the head to detect the presence and proximity of an external magnetic field. Furthermore, for those applications wherein the magnetic head is to be used as a contactless limiter switch, a highly sensitive output cannot be achieved unless the head is very closely spaced adjacent the source of magnetic field, that is, the magnet. Such very close spacing severely limits the applications of this head to contactless switching arrangements. Hence, the magnetic head does not enjoy diverse usage, nor has it been overwhelmingly successful in many commercial applications.
Many of the foregoing disadvantages attending the use of the magnetic head as a switching element have been overcome by the use of a magnetoresistive element, as disclosed in copending U.S. application Ser. No. 597,007, filed July 18, 1975, by the inventors of the present invention. As described therein, the magnetoresistive element is comprised of an insulating substrate having perpendicular, series-connected, current conducting ferromagnetic strips thereon. When this element senses a magnetic field produced by a generator magnet which is proximate thereto, an output signal is produced which is a function of the relative positions of the leading and trailing edges of the generator magnet with respect to the element. Although this device is satisfactory for many applications, there are some instances wherein it is desirable to use the magnetoresistive element to sense the proximity of a member which is not itself a magnet. But if the member to be sensed is not a magnet, the necessary magnetic field which must be detected cannot be produced.